Embodiments of the invention relate generally to structures and methods for wirebonding of power devices and, more particularly, to a power overlay (POL) structure that enables copper wirebonding of power devices regardless of the material type of the contact pads of the power device.
Power semiconductor devices are semiconductor devices used as switches or rectifiers in power electronic circuits, such as switched mode power supplies, for example. In use, power semiconductor devices are typically mounted to an external circuit by way of a packaging structure, with the packaging structure providing an electrical connection to the external circuit and also providing a way to remove the heat generated by the devices and protect the devices from the external environment. Power semiconductor devices are provided with a number of input/output (I/O) interconnections to electrically connect the device to an external circuit. These I/O connections may be provided in the form of solder balls, plated bumps, or wirebond connections. In the case of wirebond packaging, wirebonds are provided that connect bond pads or contact pads provided on the power semiconductor device to a corresponding pad or conductive element at the next level of packaging, which may be a circuit board or leadframe. Most existing power device packaging structures use a combination of wirebonds and a substrate (e.g., a direct bonded copper (DBC) substrate) to provide I/O interconnections to both sides of a respective semiconductor device. The packaging structures may be leaded (leadframe, etc.) or provided with bolted terminals for providing electrical connectivity to the packaging structure. The wirebonds form electrical connections from one surface of the packaging structure to package pins, which then interface to the external circuit, and the DBC substrate electrically couples the other surface of the packaging structure to the external circuit.
FIG. 1 depicts a wirebonded power package structure 10, according to known prior art, having a semiconductor device 12 with a gate contact pad 14 and an emitter contact pad 16 coupled to an upper surface 18 of semiconductor device 12. As shown, wirebonds 20, 22, 24 are bonded directly to the contact pads 14, 16 of the semiconductor device 12. In order to form reliable connections between the wirebonds 20, 22, 24 and the upper contact pads 14, 16 of the semiconductor device 12, the material of the wirebonds 20, 22, 24 is typically selected to match the metallization of the upper contact pads 14, 16.
A collector pad 26, often in the form of a nickel-gold metallization or a nickel-silver metallization, is formed on a lower surface 28 of semiconductor device 12. A solder 30 or sintered silver die attach material is used to couple the semiconductor device 12 to a DBC or direct bond aluminum (DBA) substrate 32.
Because power devices are typically manufactured with aluminum contact pads, the corresponding wirebonds are likewise formed of aluminum or an aluminum alloy in order to create a reliable electrical connection to the power device. Currently, there is a trend in the industry toward copper wirebonds, which provide lower electrical resistance, which leads to lower losses and higher efficiencies. However, copper wirebonds do not form reliable electrical connections to the aluminum metallization of the contact pads.
While copper contact pads could be incorporated into the power device at the time of manufacture, incorporating copper into the power device fabrication process is non-trivial and adds significant development cost and time. Also, manufacturers typically provide a single type of metallization material on all of the power devices that they manufacture. Given that a power module may incorporate power devices from multiple manufacturers, forming reliable wirebonds on those power devices is difficult because the various power devices within the given module could include dissimilar metallization materials.
Even where a power device is provided with copper metallization, coupling copper wirebonds to the copper metallization poses difficulties. For example, attaching a copper wirebond, especially a heavy gauge copper wirebond capable of withstanding high current transients, to a metallization or contact pad applies a greater amount of stress to the power device than a thinner gauge or aluminum wirebond. This is because copper to copper wirebonding requires higher energy for bonding due to its material properties compared to aluminum to aluminum wirebonding. Due to these higher energies, the wirebonding process can damage the power device.
Another issue with copper-to-copper wirebonding is the constriction of current as it flows from the contact pads of the power device to the wirebonds. The metallization layer of a contact pad on the power device is thin (e.g., a few microns) and the current must travel through this thin metallization until it encounters a wirebond and then flows through it. Wirebonds can be placed only at certain intervals due to equipment constraints, hence each power device will have only a handful wirebonds distributed across the contact pad. While providing a number of wirebonds for each contact pad helps in distributing the current flow, the resistance in the interconnect structure still results in inherent losses.
While prior attempts have been made to mitigate the above-described problems associated with copper-to-copper wirebonding, such as by optimizing the copper material properties of the contact pads and adjusting the thickness of the copper pads, there is room for further improvement in the field.
Therefore, it would be desirable to provide a POL structure that allows for the use of copper wirebonds without changing the metallization of the contact pads of the power device to copper. It would also be desirable to have a method for fabricating an I/O interconnection in the form of a wirebond that reduces device damage due to the applied stresses during the wirebonding process, thereby increasing process yield, and that provides for efficient current distribution from the power device to the wirebonds.